Systems that supply power to motors in hybrid cars etc., are equipped with boosting circuits. Reactors are used as one component of the boosting circuits. Reactors are configured by winding a coil on a core. In the case where a core is used under an alternating magnetic field, a loss called iron loss occurs in the core. Iron loss is roughly expressed as a sum of a hysteresis loss and an eddy-current loss and is particularly high when the core is used at high frequencies.
In order to decrease the iron loss in a reactor core, a core formed of a compact is sometimes used. A compact is formed by compressing coated soft magnetic powder that includes coated soft magnetic particles constituted by soft magnetic particles having surfaces coated with insulating coatings. Since the soft magnetic particles are insulated from one another by the insulating coatings, the compact has a particularly high effect of decreasing the eddy current loss.
A compact is prepared by filling a cavity formed by a relatively movable columnar-shaped first punch and a cylindrical die with coated soft magnetic powder and pressure-forming the coated soft magnetic powder inside the cavity with the first punch and a columnar second punch. Thus, the insulating coatings of the coated soft magnetic particles may become damaged due to the pressure applied during the pressure-forming and sliding contact with the die at the time the compact is withdrawn from the die. Damage on insulating coatings may cause exposure and stretching of the soft magnetic particles. As a result, the soft magnetic particles in the compact may become conductive to each other and form film-shaped conductive portions and the eddy current loss may increase thereby.
In order to decrease the eddy current loss, for example, Patent Literature (PTL) 1 describes treating surfaces of a raw compact formed by pressing coated soft magnetic powder (soft magnetic powder) with concentrated hydrochloric acid. To be more specific, the raw compact is immersed in concentrated hydrochloric acid to remove conductive portions on the entire surfaces of the raw compact to form a compact.